1. Field of the Invention
The present invention relates to a crossed-field amplifier and, more particularly, to an electron-emitting material used within a crossed-field amplifier to reduce amplifier jitter caused by stopping and restarting the amplifier.
2. Background
Crossed-field amplifiers ("CFA's") have been used for several years in electronic systems that require high RF power, such as radar systems. A CFA operates by passing an RF signal through a high voltage electric field formed between a cathode and an anode. The cathode emits electrons which interact with an RF wave as it travels through a slow-wave path provided in the anode structure surrounding the cathode. The RF wave is guided by a magnetic field, which crosses the electric field perpendicularly. Crossed-field amplifiers are disclosed in U.S. Pat. No. 4,700,109, issued Oct. 13, 1987 to MacPhail, and U.S. Pat. No. 4,814,720, issued Mar. 21, 1989, to MacPhail et al., both assigned to the common assignee, and which are incorporated herein by reference.
In some applications, it is desirable to operate the CFA in a pulsed mode in which the CFA is repeatedly turned on and off. If used in a radar system, accuracy of the pulse timing is critical to obtaining accurate return information. To start a CFA, there must exist a small number of electrons in the interaction region in order to prime the operation of the cathode. These priming electrons come from natural sources, such as residual radioactivity, electron storage from preceding pulses, cosmic rays, etc. The priming electrons impact the cathode structure causing secondary emissions of electrons from the cathode surface, further resulting in a cascade of electrons flowing in a beam through the interaction region. At relatively high pulse repetition frequencies, a large number of electrons remain in the interaction region after the CFA has been turned off. These remaining electrons prime the CFA to rapidly restart the secondary emission process. However, at low pulse repetition frequencies the electrons in the interaction region dissipate into the anode structure, leaving an absence of electrons to prime the CFA upon restart. Although the natural source electrons will eventually start the CFA, the startup time cannot be determined with certainty. Thus, the restart of the CFA at low pulse repetition frequencies is highly irregular, and is a phenomenon known as "jitter."
Several solutions to the jitter problem have been proposed. One solution involves the use of a bias circuit which holds a supply of electrons in the interaction region between the cathode and the anode when the CFA is turned off. The bias circuit is disclosed in U.S. Pat No. 4,895,586, issued Jan. 16, 1990, to Crager et al., which is assigned to the common assignee. The bias circuit supplies a negative DC voltage to the cathode which holds the electrons within the interaction region. A significant drawback of this method is that a power supply and transformer are required to supply and regulate the DC voltage. The addition of the power supply increases the complexity of the CFA, and the DC voltage must be insulated from the cathode pulse voltage, which is typically more than 10,000 volts.
Other solutions involve the use of a thermionic emitting filament disposed in a space provided between the anode vanes of the CFA. The filament thermionically emits a number of electrons in response to the application of an external low voltage. The voltage differential created by the RF wave accelerates the electrons emitted by the filament. The active filament, however, requires an external power source. Moreover, the filament's life is finite. Both of these characteristics tend to decrease the stability of the CFA.
Current solutions involve the encouragement of multipactor electron discharge in the CFA. Data suggests that such discharge occurs in the CFA due to the RF field provided by the RF input drive signal. The discharge occurs on copper surfaces of the CFA that have been lightly oxidized. Oxidization is encouraged by maintaining high oxygen levels in the CFA. Unfortunately, maintaining artificially high pressures of oxygen increases the tendency of the CFA to arc.
Accordingly, there is a need for a solution to the jitter problem that does not rely on external power sources.
Further, there is a need for a jitter solution that does not decrease the stability of the CFA.
Further, there is a need for a jitter solution that does not require artificially high partial pressures of oxygen within the CFA.